Field of the Invention
The present invention relates to a wireless relay device, a wireless communication system, and a wireless relay method.
Priority is claimed on Japanese Patent Application No. 2015-073056, filed on Mar. 31, 2015, the contents of which are incorporated herein by reference.
Description of Related Art
In a plant or a factory or the like, a distributed control system (DCS) has been implemented to realize a high level of automated operation. In the distributed control system, on-site devices (measures and actuators), which are referred to as field devices, are connected via a communication means to controllers, which control the field devices. The communication system which forms the base of such a distributed control system had been almost always one that communicates by cable. In recent years, a wireless communication system which communicates wirelessly in conformance with an industrial wireless communication standard such as ISA100.11 a or WirelessHART (registered trademark) has been realized.
Since such a wireless communication system is required to ensure the security, it is often the case that various data to be communicated via a wireless network is encrypted using cryptographic technology. For example, in the above-mentioned wireless communication system in conformance with ISA100.11a, an encryption is performed using encryption keys, which are distributed from a manager for managing a wireless network to each field device. Since the encryption keys distributed to each field device are different, a high degree of security is ensured.
Non-patent document 1 (Shuji Yamamoto and three others, “World's First Wireless Field Instruments Based on ISA100.11a”, Yokogawa Technical Report, Vol. 53, No. 2, 2010) discloses the summary of the above-mentioned wireless communication system in conformance with ISA100.11a. In addition, the Non-patent document 1 discloses a gateway in which a gateway function, a system management function (system manager), a security management function (security manager), and the like are integrated. In this gateway, distribution and update of security keys (encryption keys) is performed by the security management function.
In the wireless communication system disclosed in the non-patent document 1, the encryption keys for encrypting and decrypting data, which is communicated via a wireless network, are stored into the gateway and field devices. Therefore, devices (for example, wireless rooters) disposed in the path between the gateway and the field devices cannot decrypt and confirm the encrypted data.
In the wireless communication system disclosed in the non-patent document 1, the following two methods for allowing operators to confirm various data (measured data, data indicating device state, and the like) acquired by field devices on site are considered.
(1) First Method Using Mobile Terminal Device
In the first method, operators operate a mobile terminal device, which is communicable to a field device, to make the mobile terminal device communicate with the field device, and to display the data acquired from the field device on the mobile terminal device.
(2) Second Method Performing Data Relay to Display Device
In the second method, the data, which has been transmitted from a field device to a gateway, is relayed from the gateway to a display device (for example, a display device which is capable of performing wireless communication and is portable) and displayed on the display device.
In the first method, the operation is required in which the operators visit the installation site of the field device and operate the mobile terminal device to acquire the data from the field device. Therefore, if the operators acquire data from a plurality of field devices, it is necessary to repeat the above-mentioned operation for each field device. In addition since, in some cases, the field device is installed at a site that is difficult to reach, the above-mentioned operation is difficult to do. Thus, it is considered that the first method has a difficulty in terms of the workability.
In the second method, since it is necessary to relay the data, which has been transmitted from the field device to the gateway (or a host control system) via a wireless network, to the display device via the wireless network again, the communication resource (for example, communication band) is wastefully exhausted. In addition, if the field device or wireless router uses a battery as a power source, the consumption of the battery is accelerated. Thus, in the second method, it is considered that the communication resource and the battery are wastefully exhausted.
In addition, in the wireless communication system disclosed in the non-patent document 1, if a system failure occurs in a system manager for managing a wireless network, the field device and the like are disconnected from the wireless network managed by the system manager. In this case, in the second method, the data transmission from the field device to the gateway and the data relay from the gateway to the display device cannot be performed. Thus, in the second method, if a system failure occurs in the system manager, it is considered that the various data acquired by the field device cannot be referred on site.